Heart pacer

ABSTRACT

Device for stimulating the heart comprises means for producing stimulating pulses at regular intervals and means responsive to spontaneous heart signals for controlling the transmission of said pulses to the heart in dependence upon the timing and magnitude of the spontaneous heart signals and/or the average intramyocardial pressure. In the absence of spontaneous heart signals, stimulating pulses are produced at predetermined intervals. When spontaneous heart signals occur, an adjustable waiting period is introduced in which to allow a second spontaneous signal instead of a stimulating pulse. The duration of the waiting period is varied as a function of the interval of time which separates two consecutive spontaneous detected signals (i.e., not separated by any artificially induced heart signal).

Zacouto [111 3,857,399 [451 Dec. 31, 1974 HEART PACER' [76] Inventor:Fred Zacouto, 16, rue de la Convention, Paris l5eme, France 9/1972 Cole128/419 P Primary Examiner-William E. Kamm Attorney, Agent, orFirm-Brisebois & Kruger 57 ABSTRACT Device for stimulating the heartcomprises means for producing stimulating pulses at regular intervalsand means responsive to spontaneous heart signals for controlling thetransmission of said pulses to the heart in dependence upon the timingand magnitude of the spontaneous heart signals and/or the averageintramyocardial pressure. In the absence of spontaneous heart signals,stimulating pulses are produced at predetermined intervals. Whenspontaneous heart signals occur, an adjustable waiting period isintroduced in whichto allow a second spontaneous signal instead of astimulating pulse. The duration of the waiting period is varied as afunction of the interval of time which separates two consecutivespontaneous detected signals (i.e., not separated by any artificiallyinduced heart signal).

61 Claims, 23 Drawing Figures ST or:

1.- 0/2 f CAR l f-E A cm? AND 016 m2 'E'T03 F pa :1 RIGENEROTM M, t 1a,,

I .8 MS

J x 5M5 1* Flt-I'll 3 TI MULATO R PATENTED DEC3 1 I974 SliiU OIGF 15PATi-im guiu 1 1974 saw 030; 15

PATENTED [1533 1 4 sum on or 15 PATENTEUUECB 1 I974 sum as or 15 6E Q W6% Q PATENTEU U 1 74 sum 08 [1F 15 baht.

PATEHTEU 8 1 I914 SHEU OBBF 15 pyggmgg miss 1 I974 SHEET IOUF 15 HEARTPACER SUMMARY OF THE INVENTION This invention relates to a method andapparatus for the electrical stimulation of the cardiac muscle.

Known devices for electrically stimulating the cardiac muscle includedemand heart pacers which normally stimulate the cardiac muscle at agiven frequency by transmitting thereto stimulating impulses atpredetermined intervals corresponding to that frequency. When aspontaneous electrical signal (called a heart signal) from the cardiacmuscle occurs which is detected by the electrodes and corresponds, forexample, to a cardiac systole, the stimulation is interrupted after thissignal for a constant waiting period regardless of whether thespontaneous systole is early or late, and regardless ofthe nature ofthis systole. Such devices, while currently in use, have a number ofdisadvantages.

In particular, they offer only an unpersonalized rerhythm to beestablished when it should on the contrary provide correctiveintervention.

It is an object of the present invention to provide a process andapparatus making it possible to adapt the artificial electricalstimulation to the instantaneous condition of the cardiac muscle so asto supply a stimulating-response which takes into account whether thespontaneous systoles are or are not dangerous.

Another object of the invention is to provide a method and apparatusmaking it possible to take the cardiac muscle under control when a rapidand dangerous spontaneous rhythm tends to become established.

Another object of the invention is to provide a method and apparatusmaking it possible to let the heart beat at its own characteristicrhythm when nondangerous delayed spontaneous systoles occur.

Another object of the invention is to provide a process and apparatusfor temporarily increasing the frequency of the stimulation when adangerous rhythm tends to become established.

Another object of the invention is to differentiate the responses independence on whether the detected electrical signals are of ventricularor auricular origin.

Another object of the invention is to also provide means for adapting.the stimulating response to the hemodynamic conditions and/or theintramyocardial pressure.

Another object of the invention is to provide inexpensive and reliabledevices .which may be simply adjusted to adapt the responses of thedevices very precisely to the instantaneous state of the cardiacpatient.

Yet another object of the invention is to provide a device of verysimple construction, which is very reliable and consumes very littleelectrical energy so that it may be implanted in the body.

Other objects and advantages of the invention will appear from readingthe following description of the invention, with reference to theaccompanying drawings in which:

FIG. I is a schematic view of an electrocardiogram illustratingtheprocess according to the invention;

FIG. 2 is a general schematic view ofa device accord- 2. ing to theinvention comprising three parts A, B and C; FIG. 3 is an alternativeembodiment of the part A;

FIG. 4 is a detailed circuit'dia gram of the apparatus shown in FIG. 3;

FIGS. 5 and 6 are graphical representations illustrating the operationof the device of FIG. 3;

FIG. 7 shows another embodiment of the part A; FIG. 8 is a circuitdiagram of the delay means of FIG. 7;

FIG. 9 is a diagramatic representation illustrating the operation ofFIG. 7;v

FIG. 10 shows another embodiment of the device of FIG. 7 adapted todetect ventricular systoles;

FIG. 11 is a diagram illustrating the operation of FIG. 10;

FIG. 12 is another embodiment of the apparatus of part A;

FIG. 13 is a circuit diagram of the device shown in FIG. 12;

FIG. 14 is a diagram illustrating the operation of the device shown inFIG. 12;

FIG. 15 shows another embodiment of the part A;

FIG. [6 schematically illustrates yet another embodiment of the part A;

FIG. 17 illustrates an example ofthe function Y of K;

FIg. 18 is a circuit diagram of the part C;

FIGS. 19, 20 and 21 show a device responsive to the intramyocardialpressure;

FIG. 22 is a schematic view of another embodiment of the part A; and

FIG. 23 is a diagram illustrating the operation of the device of FIG.22. I

GENERAL DESCRIPTION OF THE PROCESS It is the object of the invention toprovide a method of electrically stimulating the cardiac muscle by meansof an automatic device comprising one or more stimulating and detectingelectrodes, in which method electrical stimulating impulses having apredetermined frequency are transmitted to the cardiac musclein theabsence of a spontaneous systole, and the transmission of a stimulatingelectrical impulse is'delayed for a predetermined waiting period when aspontaneous systole of at least a certain type occurs after astimulating im pulse. The waiting period may be of two differentdurations, at least the shortest of which is less than, or at most equalto, the predetermined period, and the choice of one of the two durationsis determined by the interval of time separating the spontaneous systolefrom the stimulating impulse which directly preceded it. The selectedduration of said waiting period is an at least partially increasingfunction of said interval of time.

FIG. 1 represents an electrocardiogram illustrating the processaccording to the invention. On this electrical cardiogram threespontaneous systoles SS are shown. After the third spontaneous systole,there is no further spontaneous systole and, in a conventional manner,the device waits a certain period of time .I. When no systole occursduring this time, the automatic device, of the demand heart pacer type,transmits a in the absence of the appearance of extrasystoles orspontaneous systoles. After the third pulse I, a new spontaneous systoleSS, is produced, which is separated from the immediately preceding pulse1 by an interval of time K,. From this moment, in accordance with theinvention, a waiting period is permitted to pass which is represented onthe drawing by Y,. The drawing also shows at I the moment at which thenext stimulating pulse would have taken place if SS, had not takenplace. If, as shown on the drawing, no new spontaneous systole appearsduring the period Y,, the apparatus emits a stimulating pulse l, at theend of this period. Since no new spontaneous systole appears after I,,the device resumes its stimulation at the intervals X by transmittingpulses I as shown on the drawing. If, after such a pulse I, for examplethe first one on the drawing, a new spontaneous systole SS appears, anew waiting period Y, is initiated and if no spontaneous systole occursduring this time, a stimulating pulse 1 is transmitted to the cardiacmuscle at the end of the delay period Y2.

In accordance with the invention, the waiting period Y (such as Y, or Ymay have any one of several different values, these values beingpreferably the smaller as the interval of time, such as K, or Kseparating the spontaneous systole from the preceding stimulating pulseis the smaller. As shown on the drawing, the value Y, is smaller thanthe value Y because the period K, is less than the interval K Moreexactly, it willbe seen on the example shown on the drawing that Y, isless than the period X, and Y, is greater than the period X. Of courseY, and Y may both be less than the interval -X. On the other hand, inaccordance with the invention, Y is an increasing function of K, when Kincreases over a part at least of its range of variation. Neverthe-,

less, in certain particular applications, Y may be a decreasing functionof K over a certain part of the range of variation ofK.

When, before the end ofthe waiting period, generally represented by Y, anew spontaneous systole takes place, a new waiting period isestablished. In a first embodiment, this new waiting period is selected,like the length of the waiting period Y, as a function, this time of theinterval of time separating the new spontaneous systole from thepreceding spontaneous systole such as SS, or $8,. In a second embodimentof the invention, this new-waiting period is, on the contrary, fixed,and preferably equal to the predetermined stimulating period (X on thedrawing). This means that if two directly consecutive spontaneoussystoles take place, the second systole is treated as a stimulatingpulse and the normal rhythm of stimulation at the predeterminedintervals X is resumed.

In one particular embodiment of the invention the different values (suchas Y, and Y of the waiting period are discontinuous, the highest valuesbeing generally attained for the longest delayed spontaneous systolesand the lowest values for the most premature systoles. Thus, by way ofexample, if the predetermined interval X is 900 milliseconds, thewaiting period Y is 600 milliseconds if the interval of duration Kseparating the spontaneous systole from the immediately precedingstimulating impulse is less than or equal to 500 milliseconds, but equalto 800 milliseconds if K is between 550 and 750 milliseconds, and equalto 1,100 milliseconds ifK is between 750 and 900 milliseconds. The threedifferent values of the waiting period are thus respectively 600, 800and 1,100 milliseconds. It will be seen that in this particular examplethe highest value of the waiting period is longer than the interval X.It could, however, be equal to or less than this period. In all cases,as has already been said, the smallest value, at least, of the waitingperiod, is less than or exactly equal to the duration of the interval X.

However, in a preferred embodiment of the invention, the differentvalues of the waiting period are arranged in a continuous manner and thewaiting period is then a continuous, generally increasing, function ofthe interval of time separating the last pulse from the consecutivespontaneous systole. This function may, however, have steps, especiallywhen the interval approaches the value of said predetermined period X,or on the contrary, when the interval K becomes very small.

Nevertheless, in certain applications, the continuous function Y of Xmay have a decreasing portion over a certain portion of the range ofvariation of K.

By way of example, FIG. 17 shows the graphical representation of thisfunction. The ordinate shows the duration Y of the waiting period beforestimulation, while the abscissa, decreased by 400 milliseconds, (forreasons of expediency hereinafter explained) shows the intervalseparating a spontaneous systole from the directly preceding stimulatingpulse. The abscissa thus shows the value K-400 milliseconds. This curvemay comprise horizontal segments, especially for the highest values ofY, which moreover, be greater than the period X.

When the largest values of the waiting period Y are greater than X, thatis to say, greater than the period of stimulation in the absence of aspontaneous systole, the value of the interval K beneath which Y is lessthan X and above which Y is greater than X lies preferably in the secondhalf of the period X which follows the stimulating pulse I directlypreceding the spontaneous systole which causes a waiting period. Thislimiting value is shown at m on FIG. 2. Naturally, if Y is a continuousfunction of K, Y is equal to X and K is equal to the distance separatingI from m.

In one embodiment of the process according to the invention the waitingperiod is always greater than the interval separating aspontaneoussystole from the immediately preceding stimulating pulse, or,in an alternative form already mentioned, from the immediately precedingspontaneous systole. This means that Y is greater than K. On thecontrary, in another form of the invention, Y is always less than K. Inthis particular embodiment, it is obviously impossible to have in theelectrocardiogram three spontaneous systoles SS separated from eachother by identical periods of time, as shown at the left of FIG. 2. Thisalternative has the advantage of insuring energetic taking in hand ofthe cardiac muscle while leaving it the least possible time within whichto produce spontaneous systoles. However, in a third embodiment, Y maybe greater than K when K is large and less than K when K is small. It isthus possible to check premature spontaneous systoles while lengtheningthe waiting period for delayed spontaneous systoles, so as to encouragethe cardiac muscle to beat at its own characteristic rhythm, but at afrequency which is not too high.

As has been seen above, the process according to the invention is put inoperation when a spontaneous systole of at least a certain type appears.By systole" is meant any electrical signal of sufficient strengthreceived on at least one of the electrodes, regardless of whether thereis or is not a contraction of the cardiac muscle. Thus, in a preferredembodiment of the invention, the process takes into account only thosespontaneous systoles which occur after the end of the refractory periodof the cardiac muscle, and the systoles which take place during thisperiod are not taken into account. This refractory period is, for anormal cardiac rhythm, for example on the order of 300 to 400milliseconds after a spontaneous systole or an electrosystole. However,according to the invention, the choices of the type of spontaneoussystole which is taken into account may also be a function either of theshape of the systole on the electrocardiogram or of the origin of thespontaneous systole. Thus, the systoles of a certain type may, forexample, be limited to ventricular extrasystoles, which are often themost dangerous.

In this .case, the detecting electrode may, in accordance with theinvention, amount to a waveform discriminator (which will be hereinafterdescribed) or a plurality of electrodes may be used which are locatedin, or in contact with, the cardiac muscle, the first electrodeexciteddetermining the origin of this spontaneous systole..

In accordance with an improved form of the invention, the stimulatingpulse normally. delivered, or delivered after a waiting period accordingto the invention, may be at least doubled or its strength may beautomatically adjusted, to increase it if a spontaneous systole appears,when the interval of time K is smaller. This assures more certainelectrical control of the cardiac muscle, the electrical conductivity ofwhich varies as a function of the instantaneous frequency of the.systoles.

In accordance with another improvement of the in-' vention the basicstimulating frequency, that is to say the predetermined period X, isincreased in a temporary manner. Y'may also be diminished simultaneouslyto adapt it to this new period X. These alternatives may be selected independence on various factors as will be seen from the following.

Thus, if it is found that, despite the variation in the waiting periodY, too large a number of spontaneous systoles are produced, thesespontaneous systoles are automatically counted, and when, for apredetermined number of systoles one has counted too large a number ofspontaneous systoles, the base period X is automatically decreased.

In another alternative, this base period varies as a function of theintramyocardial pressure detected, as will be hereinafter described. Itis also possible to detect by suitable pick-up means the hemodynamiccharacteristics so as to regulate the rhythm and shape of thestimulating pulses and to the waiting period.

GENERAL DESCRIPTION OF THE APPARATUS The invention alsorelates to adevice for carrying out i the aforesaid process and comprisesstimulating means controlled by a time base to deliver at apredetermined frequency electricalpulses for stimulating the cardiacmuscle through at least one preferably cardiac elecmost equal to thepredetermined waiting period, means for detecting spontaneous cardiacsystoles comprising at least one preferably cardiac electrode and forconducting electrical pulses corresponding to the spontaneous systolesdetected to said time delay means to initiate said waiting period, whichcorresponds to the state in which the delay means are found at thisinstant, and means to shift said time delay means successively from oneof said states into the other as a function of the time which haselapsed since tht last stimulating pulse. Said stimulating means may beany conventional means adapted to transmit a stimulating pulse so atleast one electrode when actuated by'said time base, or by said timedelay means, at the end of the waiting period.

The time base may also be of a conventional type. It is thus possible touse as a time base, for example, a capacitance which charges itselfperiodically through a load resistance and actuates, when charged to asufficiently high potential, a device such as a programmable unijunctiontransistor. It is also possible to use an astable switch ormulti-vibrator which periodically initiates the stimulation. It is alsopossible, as an alternative, to use as a time base a pulse generatoroperating at a given frequency'and associated with any suitable countingmeans, for example, a flip-flop counter or step-by-step integrator. Thiscounter when the number of predeter mined pulses has been counted,causes the emission of a stimulating pulse.

The delay means are responsive to the spontaneous systole pulses comingfrom the detecting means and may be in either'of two'different states,so as to initiate the waiting period corresponding to their state at themoment at which they are actuated, and actuate said stimulating means atthe end of said waiting period. It is thus possible to use as delaymeans two condensers (i.e., capacitors), each comprising a chargingresistor, and both connected to trigger means such, for example, as aprogrammable unijunction transistor, with one of g the condenserscharging from an initial potential slightly higherthan the initialpotential of the other condenser, but increasing in potential moreslowly the other condenser, so that the potential of this othercondenser in time overtakes thepotential of the first condenser, thusactuating a device such as the unijunction transistorpThe later thesecond condenser starts charging after the first condenser has begun tocharge, the longer the time which will be required for it to overtakethe potential of the first condenser. In this embodiment, the state ofthe delay means is thus determined by the potential which hasbeenattained by the said first condenser at the moment at which aspontaneous systolic pulse occurs to initiate charging of the secondcondenser. Of course, instead of using the charging periods of thecondensers, it is possible to use their discharge periods.

In another embodiment of the invention, said delay means may consist ofa single condenser associated with a discharge resistance. Thiscondenser is charged through a charging resistance which may be the sameas the discharge resistance, but which is preferably different. When aspontaneous systolic pulse takes place the charging of the condenser isinterrupted and the condenser discharges through the dischargeresistance up to the point at which the potential reaches a fixed valuefrom which, through appropriate means, it causes a stimulating pulse tobeproduced. It will be appreciated that the higher the chargingpotential is when this I pulse takes place, the longer the discharge tothe fixed charged during the occurrence of a spontaneous systole.

In another embodiment of the invention said time delay means compriseseveral monostable multivibrators having different periods in theunstable state, at least one of which is less than said predeterminedwaiting period. These different multivibrators are connected to aelectronic actuated rotary switch directing the spontaneous successivesystolic pulses as a function of time to the different monostablemultivibrators. The state of these time delay means is thus determinedby the position of the rotary electronic switch.

In another embodiment of the invention, the delay means comprises adigital pulse counter. Said pulse counter counts a certain number ofpulses from a pulse generator and, when it has counted the last of thesepulses, actuates the stimulating means. The number of pulses which saidcounter must count is for example, determined by another number ofpulses which has been previously counted, the state of said delay meansbeing thus determined by the number of pulses previously counted andthus by the state of the counter.

This means for changing the state of one of said delay means as afunction of the time elapsed since the last stimulating pulse may alsobe made in various ways. Thus they may comprise a source of potentialand a charging resistance associated with a capacitance. The charging ofsaid capacitance starts at the moment at which an electrical stimulatingpulse takes place, or, as an alternative, at a predetermined time aftersuch a pulse. The potential attained by said capacitance at the momentat which a spontaneous systole is detected thus determines thestate ofthe associated delay means, for example, a discharge resistanceassociated with the capacitance.

Thus when the delay means comprises two condensers, the means forchanging the delay means from one state to another comprise a source ofenergy and the charging resistance of the slower condenser. The same istrue of the variation utilizing only a single condenser.

In the variation in which said delay means comprises several monostablemultivibrators which are made successively responsive, said means forchanging the state -of one of these delay means comprises meansadvancing the rotary electronic switch at a predetermined speed from aninitial position corresponding to the occurrence of a stimulating pulse.The means for changing the time delay means from one state to anothermay also comprise digital means including a pulse generator and acounter, for example one which is automatically reset to zero at eachstimulating pulse. Beginning when a stimulating electrical pulse takesplace, the counter recommences to count the pulses which it receivesfrom the pulse generator. When a spontaneous systole occurs, the stateof the time delay means is determined by the number of pulses counted.When the time delay means also comprise a counter, this second countercounts a number of pulses determined by the number of pulses which haspreviously been counted up to that time by the first counter. The numberof pulses counted by the second counter may be, for example, the same asthe number previously counted, in which case the waiting period may beequal to or different from the interval of time separating a spontaneoussystole from the artificial electrosystole immediately preceding it. Ofcourse, in all embodiments, it is advantageously possible to use commonelectrical or electronic components, for example for the time base,and/or delay means, and/or means for changing the state of any of thedelay means.

The means for detecting spontaneous systoles comprises at least onedetecting electrode, which may be the same as the stimulating electrode,and which is adapted to detect cardiac electrical phenomena. Preferably,temporary cut-off means are provided to avoid detection of the cardiacsystole directly provoked by an articifical electrostimulation ES.Cut-off means may also be provided to prevent the detection of anycardiac electrical phenomena during a fixed time which follows astimulating pulse ES, so as to avoid taking into account phenomena whichtake place during the refractory periods of the cardiac muscle. Finally,said detecting means may comprise different selecting means making itpossible to insure the transmission to the delay means of only thesepulses received from particular systoles, either as a function of themoment at which such a systole takes place, or as a function of itsorigin (auricular or ventricular). This is made possible by utilizingtwo electrodes and determining the position of the electrode which firstdetected the electrical phenomenon of this systole, or by using meansresponsive to a characteristic of the electrical systolic pulse, such asits shape, length, amplitude, etc.

The device which has been described may take several forms. It may bethat a spontaneous systole detected which has triggered the time delaymeans will be followed by a second spontaneous systole before the end ofthe waiting period. In a first embodiment of the invention means areprovided to return the different electronic components of the device totheir original state so that a second systole is considered by thedevice exactly like an artificial stimulating pulse.

In another variation of the invention however, means may be provided toagain trigger the time delay means to provide a waiting periodcorresponding to the state in which said provide a waiting periodcorresponding to the state in which said time delay means is found atthe moment at which the second systole takes place. In this case saidtime delay means are preferably divided into two parts as will behereinafter seen.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A. FIRST EMBODIMENTThis first embodiment relates to a device comprising two condenserswhich are charged at two different speeds, the potential of onecondenser being adapted to overtake the potential of the othercondensenThis device is described with reference to FIGS. 2, 3, 4, 5

and 6.

Referring first to FIG. 2, the device according to the inventioncomprises three parts, A, B and C, of which only parts A and B will nowbe described. Part C is hereinafter described since it is notindispensable to the operation of parts A and B. Part B comprises anintracardiac catheter I which terminates in several electrodes 2,certain of which are positive, while others are negative. Theseelectrodes 2 act both as detectors and as stimulators for the cardiacmuscle. The electrodes 2

1. Method of electrically stimulating the cardiac muscle by means of anautomatic device comprising heart stimulating and detecting means, whichmethod comprises the steps of: A. detecting spontaneous heart signals,B. transmitting electrical stimulating pulses to the cardiac muscle atpredetermined intervals in the absence of a spontaneous heart signal, C.sensing the time between the occurrence of an electrical stimulatingpulse and the detection of a directly successive spontaneous heartsignal, D. suspending the transmission of an electrical stimulatingpulse for one of at least two waiting periods when a spontaneous heartsignal is detected after a stimulating pulse, the length of at least oneof said at least two waiting periods being no greater than saidpredetermined inverval, and E. selecting said one of at least twowaiting periods in dependence upon said sensed time, the length of saidselected waiting period being an increasing function of said sensed timeover at least part of the range of variation of said sensed time.
 2. Amethod according to claim 1 comprising the step of initiating a fixedfurther waiting period upon the detection of a spontaneous heart signaldirectly consecutive to a previous spontaneous heart signal.
 3. A methodaccording to claim 2 in which said fixed further waiting period is equalto said predetermined interval.
 4. A method according to claim 1comprising the steps of identifying those spontaneous heart signalswhich are of ventricular origin and initiating one of said waitingperiods upon the detection of a heart signal of ventricular origin.
 5. Amethod according to claim 4 which comprises the step of initiating awaiting period upon the detection of a heart signal of non-ventricularorigin, the length of said waiting period for a heart signal ofnon-ventricular origin being different from that for the heart signalsof ventricular origin.
 6. Method as claimed in claim 1 which furthercomprises the steps of periodically counting all heart signals up to apredetermined total number, simultaneously counting the number of atleast that portion of said spontaneous heart signals which are of anidentifiable type, and temporarily decreasing the length of saidpredetermined intervals when the number of dangerous heart signalscounted is greater than a predetermined number, said dangerous heartsignals counted being those which take place after a period of time atleast as great as the refractory period of the cardiac muscle and beforethe elapse of a fixed period greater than half of said predeterminedinterval.
 7. Process as claimed in claim 6 in which said identifiablesignals are those of dangerous typE and only said dangerous signals arecounted simultaneously.
 8. Process according to claim 6 in which twelveheart signals are counted and said predetermined regular interval isdecreased when the number of dangerous heart signals counted during thetime required to count said twelve heart signals is greater than anumber between 1 and 3 inclusive.
 9. A method according to claim 1further comprising the steps of: A. sensing the time between thedetection of a spontaneous heart signal and the detection of a directlysuccessive second spontaneous heart signal, B. suspending thetransmission of an electrical stimulating pulse for one of at least twodifferent further waiting periods when said directly successive secondspontaneous heart signal is detected and C. selecting said one of atleast two further waiting periods in dependence upon said sensed timeimmediately preceding the detection of said directly successive secondspontaneous heart signal, the length of said selected waiting periodbeing an increasing function of the sensed time preceding the detectionof said directly successive second heart signal over at least part ofits range of variation.
 10. A method according to claim 9 wherein saidtime subsequent to the detection of a spontaneous heart signal ismeasured by dividing it into at least two consecutive time durations.11. A method according to claim 1 wherein said time consecutive to theoccurrence of an electrical stimulating pulse is measured by dividing itinto at least two consecutive time durations.
 12. A method according toclaim 11 in which said time subsequent to the occurrence of astimulating pulse is divided into a plurality of consecutive identicaltime durations, each of which corresponds to a particular length of saidwaiting period.
 13. A method according to claim 12 in which said timesubsequent to the detection of a spontaneous heart signal is dividedinto a plurality of consecutive identical time durations, each of whichcorresponds to a particular length of said waiting period.
 14. A methodas claimed in claim 1 comprising the further step of avoiding saidsuspension of a transmission of an electrical stimulating pulse when aspontaneous heart signal is detected in a fixed period following eachstimulating pulse, said fixed period being greater than the refractoryperiod of the cardiac muscle.
 15. A method as claimed in claim 14 inwhich said fixed period is between 200 and 400 milliseconds.
 16. Methodas claimed in claim 1 which comprises the step of sensing the timebetween two directly consecutive heart signals and initiating after thelast of said sensed heart signals a variable period before transmittingthe next stimulating pulse, said period increasing when said timeinterval increases and decreasing when said time interval decreases. 17.A method as claimed in claim 1 which comprises the steps of sensing theaverage value of the intramyocardial pressure and varying at least oneof a. the intensity of the stimulation, and b. the relationship betweensaid waiting period and said interval of time in dependence on saidaverage value.
 18. In a heart pacing method for electrically stimulatinga cardiac muscle by means of an automatic device comprising heartstimulating and detecting means, said method comprising the steps ofdetecting spontaneous heart signals, starting a waiting period after thedetection of a spontaneous heart signal, producing an electricallystimulating pulse at the end of said waiting period when no furtherspontaneous signal has been detected within said waiting period, andtransmitting said stimulating pulse to the cardiac muscle, theimprovement comprising the steps of: A. measuring the time subsequent tothe detection of a spontaneous heart signal, and B. adjusting the lengthof said waiting period in dependence upon the measured time whichcorresponds to the detection of the corresponding spontaneous heartsignal, the adjustEd value of said waiting period being an at leastpartially increasing function of said measured time.
 19. An improvementas claimed in claim 18 wherein said time subsequent to the detection ofa spontaneous heart signal is measured by dividing it into a pluralityof consecutive time durations.
 20. An improvement as claimed in claim 18further comprising the step of avoiding said suspension of transmissionof an electrical stimulating pulse when a heart signal is detected in afixed period following the previous detected spontaneous heart signal,said fixed period being greater than the refractory period of thecardiac muscle.
 21. An improvement as claimed in claim 18 furthercomprising the step of adjusting the length of said waiting periods tobe shorter than said measured time.
 22. An improvement as claimed inclaim 18 further comprising a step of avoiding the adjustment of thelength of said waiting period when the corresponding spontaneous heartsignal occurs in a fixed period following a previous detectedspontaneous heart signal and being greater than the refractory period ofthe cardiac muscle.
 23. A demand type heart pacer comprising: A.detection and stimulation leads adapted to be connected to a heart, B.detecting means for detecting spontaneous heart signals, said detectingmeans being connected to said detection leads, C. pulse generator meansconnected to said stimulating leads and providing a stimulating pulseupon control thereof, D. pulse generator control means connected to saidpulse generator means and including a time-base circular means operableto cause stimulating pulses from said pulse generator means atpredetermined regular intervals in the absence of detected spontaneousheart signals, said time base circuit means being responsive to saiddetection means to cause staggering of said period upon detection of aspontaneous heart signal, said pulse generator control means furthercomprising: D1. time sensing means for sensing a time interval between astimulating pulse and a successive detected spontaneous signal D2. timedelay means for providing a variable waiting period after detection of aspontaneous heart signal, said time delay means being connected to saidtime sensing means and being responsive thereto for giving to saidwaiting period a value which is an at least partially increasingfunction of said time interval sensed by said time sensing means, one atleast of said values being at most equal to said regular intervals, saidtime sensing means and time delay means being both connected to saiddetection means.
 24. Device as claimed in claim 23 in which said timedelay means comprises: A. a single capacitor (C, C3) associated with adischarge resistance and a trigger (PUT) connected to trigger thestimulation when the discharge potential of said capacitor attains apredetermined value, B. means for changing said delay means into atleast two different states for each of which said waiting period has adifferent value comprising a charging resistance associated with saidcapacitor to charge said capacitor from its initial potential, and C.means (A202, A111, A222, Q3, Q4) to interrupt the charging and commencethe discharging when a spontaneous heart signal of at least a certaintype takes place, said capacitor being associated with trigger means(116) connected to trigger the stimulation when the charging potentialof said capacitor has reached a predetermined potential.
 25. Device asclaimed in claim 24 in which said trigger means is responsive to thefinal value of the potential attained by said capacitor for triggeringthe transmission of a stimulating pulse and thus constitutes the timebase.
 26. Device as claimed in claim 25 in which said trigger means is aprogrammable unijunction transistor.
 27. Device as claimed in claim 26comprising means (121) for rapidly discharging and begiNning therecharging of said capacitor when a spontaneous heart signal takes placeduring the discharge.
 28. Device according to claim 23 which comprises abistable means (FF100) connected, on the one hand, to the detectingmeans to be switched into one position and, on the other hand, to atrigger (116) responsive to the charging and discharging potentials ofthe capacitor to be switched into another position when the potential ofthe capacitor becomes less than a predetermined value during itsdischarge, said bistable means being connected to cause the slowdischarge of said capacitor through its discharge resistance whenswitched by the detecting means and cause the rapid discharge of saidcapacitor and the transmission of a stimulating pulse when switched bysaid trigger.
 29. Device according to claim 28 in which said bistablemultivibrator (FF100) is also connected to one input of a first gate(ET104), the other input of which is connected to the detecting means toopen said gate during the discharge of said capacitor into its dischargeresistance, said gate being connected to means (121) to rapidlydischarge said capacitor without the transmission of a stimulatingpulse, upon the detection of a spontaneous heart signal during the timesaid bistable multivibrator is in the position assuring the slowdischarge of said capacitor.
 30. Device according to claim 29 comprisingmeans actuated by the detecting means to connect said bistable means toa second gate (ET102) connected on the one hand to the detecting meansand on the other hand to gate actuating monostable means (MS102) foropening said second gate during a period less than said predeterminedinterval, said means also comprising a third gate (ET103) connected toopen at the end of the open period of said second gate (ET102), theoutput of said third gate (ET103) being so connected to said device forrapidly discharging said capacitor as to abruptly discharge saidcapacitor without the transmission of a stimulating pulse when saidthird gate is open.
 31. Device as claimed in claim 30 in which saidthird gate (ET103) is connected to said means for rapidly dischargingthe capacitor without the transmission of a stimulating pulse throughmonostable delay means (MS103) adapted to actuate said means (121) aftera fixed time.
 32. Device according to claim 30 in which the detectingmeans are connected to a fourth gate (ET100), said fourth gate beingclosed after a stimulating pulse for a period at least as great as therefractory period of the cardiac muscle by monostable means which istemporarily actuated by the stimulating pulse and connected totemporarily switch said gate actuating monostable means (MS102),temporarily opening first said first gate leading to said bistablemeans, and closing said third gate (ET103), after which said third gate(ET103) is opened, and said first gate leading to said bistable means isclosed.
 33. Device according to claim 24 which comprises means connectedto the detecting means for selecting those heart signals of ventricularorigin and directing them to the time delay means (HV1), said selectingmeans comprising a trigger (246) supplying a square wave signal when thedetection potential resulting from the presence of a heart signalexceeds a predetermined value (U), said trigger being connected toswitch, at the beginning of said square wave signal, monostable means(MS207) supplying, while switched, a potential to means (C201, D201,R201) which inhibits the transmission toward the time delay means of aterminal pulse when said monostable is not returned to its stableposition, and permits the transmission of said terminal pulse from thetrigger to said time delay means when the end of said signal supplied tosaid trigger takes place after the return of said monostable means toits stable position.
 34. Device as claimed in claim 33 which comprisessecond time delay means for determining waiting periods different fromthose of said first mentioned time delay means, said second time delaymeans being connected to inhibiting means (C202, R202, D202) actuated bythe potential transmitted by said monostable means to permit thetransmission of the terminal pulse of said trigger to said second timedelay means when said monostable has not returned to its stable positionand prevent said transmission to the second time delay means when saidterminal pulse from said trigger takes place after the return of saidmonostable to its stable position.
 35. Device as claimed in claim 23 inwhich said time delay means comprises a plurality of monostablemultivibrators having different unstable periods, at least one of whichis shorter than said predetermined interval, said means for changingtheir state rendering said multivibrators successively responsive to thedetecting means.
 36. Device as claimed in claim 35 in which said meansfor changing state comprises a plurality of gates, the output of eachgate leading to one of said monostable multivibrators, each of saidgates having an input connected to said detecting means and anotherinput connected to means for successively opening said gates.
 37. Deviceas claimed in claim 23 in which said time delay means comprise a digitalpulse counter (C400), a pulse generator being connected to supply pulsesto said pulse counter from the moment at which a spontaneous heartsignal takes place, and a memory determining the number of pulses to becounted prior to said stimulation.
 38. Device as claimed in claim 37which comprises at least one pulse generator (414), an adder-subtracter(C400) connected to said pulse generator, means for initiating thestimulation during the return to zero of said adder-subtracter, andcontrol means (FF400) bringing said adder-subtracter into countingposition at each stimulating pulse and switching said adder-subtracterto subtracting position when a spontaneous heart signal takes place. 39.Device as claimed in claim 38 in which said central means comprises aflip-flop (FF400) responsive to the detecting means.
 40. Device asclaimed in claim 39 in which said flip-flop connects saidadder-subtracter to an oscillator (412) during said subtraction and saidoscillator is connected to transmit the subtracting pulses thereto at aninstanteous frequency different from the frequency of its adding pulses.41. Device as claimed in claim 23 in which said time delay meanscomprises a pair of time delay devices connected to said detecting meansby an alternating electronic directing means responsive to thespontaneous heart signals, a first one of said time delay devices beingconnected to initiate a variable waiting period promptly upon thedetection of a spontaneous heart signal, while said heart signalsimultaneously actuates means for changing the state of the other timedelay means to permit it to initiate a variable waiting period when anew spontaneous heart signal is detected before the end of the waitingperiod of said first time delay device.
 42. In a device as claimed inclaim 23, the improvement which comprises first means for periodicallycounting a predetermined number of detected heart signals, second meansfor simultaneously counting the number of spontaneous heart signals, andmeans responsive to second counting means for temporarily decreasing thepredetermined interval between stimulating pulses, said first countingmeans being connected to reset said second counting means to zero uponthe return of the first counting means to zero.
 43. Device as claimed inclaim 42 comprising a bistable means and means for transmitting aswitching pulse to said means when said second counting means havecounted a number of spontaneous heart signals greater than thepredetermined number, means connecting said bistable means to said fiRstcounting means to reset it to zero when said bistable means is switchedby said second counting means and means connecting said bistable meansto said means for temporarily decreasing said predetermined interval ofstimulation.
 44. Device as claimed in claim 43 in which said means fortemporarily decreasing said predetermined interval comprise a time basetransmitting cardiac stimulating pulses at regular intervals less thansaid predetermined interval and control means (518) for alternatelyinitiating and interdicting the operation of said time base, saidcontrol means being themselves actuated by the switching of saidbistable means and being connected in turn to a second input of saidbistable means to switch back said bistable means at the moment ofinterdiction of said time base.
 45. Device as claimed in claim 44 inwhich said control means (518) are also connected to said secondcounting means to reset it to zero at the end of the duration of theoperation of said time base (520).
 46. Device according to claim 45 inwhich said second counting means are connected to means for identifyingand selecting dangerous spontaneous heart signals which take place aftera period of time at least as great as the refractory period of thecardiac muscle and before the elapse of a fixed period greater than halfof said predetermined interval.
 47. Device according to claim 23 inwhich the detecting means comprise a device responsive to theintramyocardial pressure including an intramyocardial element having apressure detector surrounded by a liquid enclosed in a flexibleenvelope, said pressure detector being connected to at least oneconductor leading to means responsive to the average pressure detectedto strengthen the stimulation in the case of an increase in said averagepressure.
 48. Device as claimed in claim 47 comprising anintraventricular catheter in which said pressure detector is positionedin a trans-septal branch of said catheter.
 49. Device as claimed inclaim 48 in which said envelope is made of cloth having a very finemesh.
 50. Device as claimed in claim 47 in which said envelope is animpermeable membrane.
 51. Device as claimed in claim 47 in which saidliquid is polyvinyl pyrrolidone.
 52. The device of claim 23 wherein saiddetection means comprises inhibition means for preventing transmissionto said pulse generator control means after each stimulating pulseduring a fixed period at least equal to the refractory period of theheart.
 53. The device of claim 23 wherein said time delay means isdivided into at least two different circuit means each of which, whenselectively actuated, provides a waiting period different from thewaiting periods of the others of said circuit means, said detectionmeans comprising at least two different gate means, each gate meansbeing connected to one of said circuit means, amd gate control meansresponsive to said pulse generator means for opening said gatessuccessively after occurrence of a stimulating pulse, each of said gatesbeing closed when the successive gate is opened.
 54. Devices as claimedin claim 53 in which said gate control means comprises two monostablecircuits connected to each other to switch successively one after theother.
 55. Method of electrically stimulating the cardiac muscle bymeans of an automatic device comprising heart stimulating and detectingmeans, which method comprises the steps of: A. detecting spontaneousheart signals, B. transmitting electrical stimulating pulses to thecardiac muscle in the absence of a spontaneous heart signal, C. sensingthe time between the occurrence of an electrical stimulating pulse andthe detection of a directly successive spontaneous heart signal, D.suspending the transmission of an electrical stimulating pulse for oneof at least two different waiting periods, when a spontaneous heartsignal is detected after a stimulating pulse, E. selecting the length ofsaid one waiting period in Dependence on said sensed time, the length ofsaid waiting period being shorter than said sensed time.
 56. A methodaccording to claim 55 further comprising the steps of A. measuring thetime consecutive to the detection of a spontaneous heart signal, B.suspending the transmission of an electrical stimulating pulse for oneof at least two different waiting periods, when a spontaneous heartsignal is detected after a previous spontaneous heart signal, C.selecting the length of said waiting period in dependence upon the timemeasure which corresponds to the detection of said spontaneous heartsignal, the selected value of said waiting period being shorter thansaid measured time as an increasing function of said measured time. 57.In a demand type heart pacer comprising detection and stimulating leadsadapted to be connected to a heart, detection means for detectingspontaneous heart signals, said detection means being connected to saiddetection leads, pulse generator means for providing, when actuated, astimulating pulse to said stimulator leads, and pulse generator controlmeans responsive to said detection means to start a waiting period onoccurrence of each detected spontaneous heart signal before controllingsaid pulse generator means to cause a stimulating pulse, the improvementwherein said pulse generator control means comprises a time delay meansdivided in at least two different circuit means which, when selectivelyactuated, each provides a waiting period different from the waitingperiods of the others of said circuit means, said detection meanscomprising at least two different gate means, each gate means beingconnected to one of said circuit means, and gate control meansresponsive to said pulse generator means for opening successively saidgates after occurrence of a stimulating pulse, each of said gates beingclosed when the successive gate is opened.
 58. A demand type heart pacercomprising: A. detection and stimulation leads adapted to be connectedto a heart, B. detection means for detecting spontaneous heart signals,said detection means being connected to said detection leads, C. pulsegenerator means connected to said stimulating leads and providing astimulation pulse upon control thereof, D. pulse generator control meansconnected to said pulse generator means and comprising time delay meansproviding a base waiting period after occurrence of a stimulating pulseand operating said pulse generator means at the end of said base waitingperiod when no spontaneous heart signal has been detected during saidbase waiting period, said time delay means comprising time sensing meansconnected to said detection means and responsive to the time intervalseparating a stimulation pulse from a directly successive spontaneousheart signal, for causing said time delay means to provide, upondetection of a spontaneous heart signal, a variable waiting period, thevalue of which is an at least partially increasing function of said timeinterval sensed by said time sensing means, one at least of said valuesbeing at most equal to the length of said base waiting period.
 59. Thedevice of claim 58 wherein: A. said time sensing means comprises: A1. afirst capacitor, A2. a charging resistance connected to said firstcapacitor, A3. means for charging said first capacitor through saidcharging resistance, and B. wherein said time delay means furthercomprises: B1. a second capacitor, B2. a charging resistance connectedto said second capacitor, B3. means for charging said second capacitorthrough said charging resistance, B4. a triggering means connected tosaid first and second capacitors and responsive to a predeterminedpotential difference between said first and second capacitors to cause astimulating pulse from said pulse generator means, and B5. meansconnected To said detection means for resetting said second capacitor onthe occurrence of a spontaneous heart signal detection, C. there beingfurther provided means for resetting said first and second capacitors onthe occurrence of a stimulating pulse and inhibiting charging of saidsecond capacitor for a fixed time interval on the occurrence of saidstimulating pulse.
 60. The device of claim 59 comprising: A. a first anda second gate connected to said detection means, B. gate control meansresponsive to said pulse generator means for opening successively saidfirst gate and said second gate after occurrence of a stimulating pulse,said first gate being closed when said second gate is being opened, saidfirst and second gates being connected to said second capacitor forresetting said capacitor on occurrence of a spontaneous heart signalwhen one of said gates is in opened condition, and said second gatebeing connected to a further means for temporarily inhibiting chargingof said second capacitor on occurrence of a spontaneous heart signalwhen said second gate is in opened condition.
 61. The device of claim 60in which said gate control means comprises at least two monostablecircuits connected to each other to switch successively one after theother.